(1) Field of the Invention
The present invention relates to a PCM coder and decoder having the function of two-wire/four-wire conversion, and more particularly to a coder and decoder (CODEC) which is used in the subscriber's circuit of a digital switching system, etc. and in which a PCM signal applied to a four-wire receiving line is decoded and converted into an analogue signal that is sent to a two-wire subscriber's line, while an analogue signal from the two-wire subscriber's line is encoded into a PCM signal that is sent to a four-wire transmission line.
(2) Description of the Prior Art
The subscriber's circuit of a switching system is arranged so as to have the functions of battery feed, overvoltage protection, ringing, supervision, PCM encoding and decoding, two-wire/four-wire conversion, test, etc.
In the arrangement having these functions, a circuit for the two-wire/four-wire conversion has heretofore been constructed separately from a PCM coder and decoder. Recently, however, with progresses in semiconductor integration technology and signal processing technique, it is studied to construct the conversion circuit unitarily with the PCM coder and decoder (Electronics/May 5, 1982, pp. 113-118). In order to realize the function of two-wire/four-wire conversion by the use of an electronic circuit, an input signal from a four-wire receiving line must be sent to only a bidirectional two-wire line, such as subscriber's line, so as not to return to a four-wire transmission line and then be provided as an output. To this end, in a PCM coder and decoder hitherto proposed, a circuit for cancelling a return signal, namely, a balancing circuit is constructed of a digital circuit unitarily with the PCM coder and decoder. More specifically, a voice analogue signal from a two-wire line to be transmitted is converted into a digital signal via a pre-filter for eliminating high frequency noise and an A/D converter, and the digital signal has its band limited by a digital filter and is thereafter delivered to a four-wire transmission line as a PCM signal. On the other hand, a PCM signal received from a four-wire receiving line has its band limited by a digital filter and is passed through a D/A converter and a post-filter so as to be supplied to a two-wire line as a voice analogue signal. The balancing circuit is so constructed that a filter which has characteristics approximating the transmission characteristics of the path of the return signal is interposed between the A/D converter and the D/A converter so as to subtract the output of the filter from the output of the A/D converter.
With the PCM coder and decoder as stated above, an amplifier circuit is generally provided in the path of the return signal. It is therefore sometimes the case that the return signal becomes greater in amplitude than the received signal and exceeds the coding level of the A/D converter. Even when the return signal is not higher than the maximum coding level, the return signal is superposed on the signal from the two-wire line which ought to be transmitted, resulting in the problem that the dynamic range of the transmission signal becomes insufficient to deteriorate the S/N ratio.
Further, when it is intended to implement the coder and decoder in the form of an LSI, the composite amplitude of the return signal and the transmission signal might exceed a supply voltage to destroy the LSI.
In contrast to the aforementioned balancing circuit realized by the all-digital circuit, it is also considered that a balancing circuit is constructed of only analogue circuits and is formed between the input of an A/D converter and the output of a D/A converter. Since, however, the balancing circuit needs to be suited to various loads, namely, impedances on the two-wire line side, it must comprise a plurality of analogue circuits having different transfer functions, and a circuit for selecting and controlling the optimum analog circuit is required. In case of digital circuitry, a plurality of balancing circuits can be readily realized by changing the coefficient of a multiplier unit and without adding any circuit device, whereas in case of analogue circuitry, the realization of a plurality of balancing circuits of different transfer functions makes it necessary to switch and use different resistors, capacitors, operational amplifiers etc., resulting in a large circuit device scale which incurs the problem that an economical occupation area cannot be held in the LSI implementation.
Further, a circuit arrangement comprising an analogue balancing circuit and a digital balancing circuit has been proposed (for example, "The Bell System Technical Journal", Vol. 60, No. 7, pp. 1585-1619, September 1981).
The analogue balancing circuit proposed, however, needs to especially employ a transfer function having a biased frequency characteristic with one pole and no zero so that the impedance of a two-wire/four-wire interface portion (for example, a transformer) can be coped with besides a plurality of two-wire subscriber's line impedances. Accordingly, the realization of such an analogue balancing circuit requires a capacitor or an inductor of comparatively large element value, and the LSI implementation is, in effect, impossible from the economical viewpoint as in the foregoing case. Moreover, with the analogue balancing circuit, a fluctuation in the absolute value of the element value directly changes the frequency--gain characteristic and phase characteristic of a return signal, and it becomes very difficult to suppress the return signal precisely by means of the digital balancing circuit at the suceeding stage.